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Enhertu Shows Activity in HER2-Ultralow Breast Cancer as Pathologists Grapple With Testing

ASCO crowd

NEW YORK – In the DESTINY-Breast06 trial, AstraZeneca and Daiichi Sankyo's Enhertu (trastuzumab deruxtecan) demonstrated activity in breast cancers with ultralow HER2 expression, creating a new subclass of patients and further complicating reporting this biomarker.

The Phase III trial, data from which were reported at the American Society of Clinical Oncology's annual meeting on Sunday, included 866 patients with previously treated hormone receptor (HR)-positive, HER2-low or -ultralow metastatic breast cancer who were randomized to receive either the antibody-drug conjugate Enhertu or physicians' choice of chemotherapy. Researchers led by Giuseppe Curigliano, professor of medical oncology at the University of Milan, reported at the meeting that Enhertu delayed disease progression compared to chemotherapy in breast cancer patients with ultralow HER2 expression in this trial.

In 2022 and 2023, Enhertu was approved in the US and Europe, respectively, for patients with unresectable or metastatic HER2-low breast cancer, based on results from the Phase III DESTINY-Breast04 trial. In this study, HER2-low was defined as an immunohistochemistry staining result of 1+, or if patients were IHC 2+, and they also had to have negative results for HER2 gene amplification by in situ hybridization (ISH).

Before Enhertu's approval in this setting, patients were considered either HER2-positive or -negative. HER2-positive tumors are eligible for HER2-targeted therapy, such as Genentech's Herceptin (trastuzumab), and have HER2 expression scores of IHC 3+, or are IHC 2+ and ISH positive. HER2-negative tumors with IHC 1+ or 0 scores are deemed HER2-negative and are ineligible for HER2-targeted treatment.

Enhertu's approval in HER2-low breast cancer made a new subset of patients, previously ineligible for HER2-targeted therapy, eligible for such treatment. However, it had oncologists and pathologists wondering how low a patient's tumor HER2 expression could be and still be responsive to Enhertu.

AstraZeneca and Daiichi Sankyo explored this question in DESTINY-Breast06. In April, they said they would discuss the results from this study with regulators to potentially expand the earlier HER2-low breast cancer indication to include patients with ultralow HER2 expression.

Following the latest readout from DESTINY-Breast06, experts at ASCO's annual meeting said that the challenge will be how to identify these patients from true HER2-negative patients using current IHC tests while more sensitive assays are developed and reporting guidelines are updated.

"The most important thing is that our pathologists now need to identify this new class of patients," Curigliano said. "What I will ask my pathologist is at least to report if any staining is present in a HER2 0 [result]. That is enough, in my opinion, to decide to give trastuzumab deruxtecan or not without modifying [current guidelines]."

According to joint guidelines from ASCO and the College of American Pathologists, a tumor gets an IHC 2+ or equivocal score for HER2 expression if there is weak to moderate complete membrane staining in more than 10 percent of cells. In this case, a reflex ISH test must be ordered to determine if the tumor is HER2-positive or -negative. Pathologists should score a tumor IHC 1+ when there is incomplete membrane staining that is faint or barely perceptible in greater than 10 percent of cells and give an IHC 0 score when there is no staining or incomplete, faint membrane staining in 10 percent or fewer tumor cells.

Similar efficacy in ultralow HER2

In the DESTINY-Breast06 trial, the researchers compared the activity of Enhertu versus chemo in breast cancer patients with both HER2-low and -ultralow tumors, and in the HER2-low and -ultralow groups separately. The study included 713 HER2-low patients (82 percent) and 153 HER2-ultralow patients (18 percent).

As in the earlier DESTINY-Breat04 trial, here, HER2-low was defined as a HER2 expression score of IHC 2+ and a negative ISH test or an IHC 1+ score. The additional HER2-ultralow category was defined in DESTINY-Breast06 as a HER2 expression score of greater than 0 and less than 1 or an IHC 0 result with membrane staining. In DESTINY-Breast06, Curigliano noted that HER2-low status was investigator assigned while the HER2-ultralow status was centrally confirmed.

To be eligible for the trial, patients had to have previously gotten either two prior lines of endocrine therapy or endocrine therapy with a CDK4/6 inhibitor, but they could not have had prior chemotherapy for advanced or metastatic disease. This study explored an earlier line of treatment than DESTINY-Breast04, which evaluated Enhertu in patients who had received chemo.

In the overall study population (comprising HER2-low and -ultralow groups), the median progression-free survival on Enhertu was 13.2 months compared to 8.1 months on chemo. Median progression-free survival remained the same in the HER2-low and -ultralow subgroups when broken out, 13.2 months, suggesting that the ultralow group derived as much benefit from Enhertu as those with higher HER2 expression.

Overall survival data were not fully mature as of this analysis, but the 12-month survival rate in the overall study population was 87 percent on Enhertu versus 81.1 percent on chemo. The 12-month survival rate among HER2-low patients was 87.6 percent and 84 percent among HER2-ultralow patients.

"Data in the HER2-ultralow [patients] were consistent with the HER2-low population," Curigliano concluded at the meeting. "DESTINY-Breast06 establishes trastuzumab deruxtecan as an effective new treatment option for patients with the HR-positive HER2-low and HER2-ultralow metastatic breast cancer following one line or more of endocrine-based therapy."

HER2-ultralow testing challenges

If AstraZeneca and Daiichi Sankyo are successful in expanding Enhertu's indication to HER2-ultralow breast cancer, the challenge will be to distinguish between HER2-ultralow and true HER2-negative patients, ASCO Chief Medical Officer Julie Gralow said during a press conference that highlighted this data.

"Within HER2 IHC 0, now we're going to have two classifications," Gralow said. "We're going to have the ultralow group with less than 10 percent of cells that have a little bit of membrane staining, and then, what we're calling true zero. We're going to have to work with our pathology colleagues, because we might not have a good discriminator when [HER2 expression] is that low."

After Enhertu's HER2-low approval, oncology and pathology experts began calling for clearer parameters to define the HER2-low and HER2-ultralow groups. Shortly after the US Food and Drug Administration approved Enhertu, Roche's Pathway Anti-HER2/neu (4B5) Rabbit Monoclonal Primary Antibody test was approved as a companion diagnostic to identify HER2-low patients. The same test, marketed as the Ventana HER2 (4B5) Rabbit Monoclonal Primary Antibody RxDx, received a CE mark in Europe this year. This test is designed to be used with Roche's BenchMark IHC/ISH slide staining instrument and includes a scoring algorithm to help pathologists identify low HER2 expressors.

Hospitals and labs that have barely had the chance to adjust their diagnostic workflows to reliably measure HER2-low are now confronted with further changes to the biomarker requirements for guiding HER2-targeted therapy. Michael Misialek, associate chair of pathology at Mass General Cancer Center at Newton-Wellesley Hospital, said in an interview that the research is advancing so quickly that pathology guidelines haven't even reconciled HER2-low reporting before data on HER2-ultralow patients came out. "We're jumping even further ahead by looking at HER2-ultralow when we haven't fully embraced consistent reporting on HER2-low," he said.

In a 2023 update to their HER2 testing guidelines, ASCO and CAP acknowledged the challenges of assessing HER2-low by IHC. They did not adopt any new reporting standards citing uncertain evidence but noted that pathologists could add a footnote to their reports that patients with an IHC 1+ or 2+ result, who may have been considered HER2-negative prior to the approval, may now be eligible for treatment with Enhertu.

AstraZeneca and Daiichi Sankyo are also aiming to establish the lowest HER2 expression threshold at which patients still derive benefit from Enhertu in the DESTINY-Breast15 trial, which is enrolling patients who are HER2-low and HER2 IHC 0.

However, IHC testing is "relatively poor at distinguishing HER2-low and -ultralow cancers," said Ian Krop, chief clinical research officer at Yale Cancer Center, who discussed the DESTINY-Breast06 results at ASCO.

"The original IHC test was designed to distinguish very high levels of HER2, the 3+ patients from all the lower levels," Krop explained. "If you ask the test to do this, it's essentially like asking the test to weigh mice on a scale made for elephants. It's probably not going to work."

Kimberly Allison, director of breast pathology at Stanford Medicine, said in an email that technical issues in the IHC testing process could also pose challenges when gauging low HER2 expression. She said differences in pre-analytic factors, for example, the time elapsed before the cancer tissue sample is fixed and how well it is fixed, could produce a false-negative, while the way that individual pathologists' run the test and interpret the results could muddle detection of very low levels of HER2.

"In essence, cancer samples with no staining may just be below the limit of detection of the test," Allison said. "Ideally, a clinically validated limit of detection control sample would also be available so pathologists could ensure that their assays are sensitive enough in their labs each time the test is run."

Allison also suggested that trials should include IHC 0 patients who are ultralow and those with no staining "to help determine if we can offer trastuzumab deruxtecan to patients regardless of HER2 status."

While there are challenges with present IHC testing, Krop said that there are new tests being developed to fill this gap, such as better IHC protein assays, HER2 mRNA assays, and tests that assess the spatial distribution of HER2 expression. These new assays will need to be validated for use alongside Enhertu, he added.

Another challenge for pathologists, according to Misialek, is that they may not have enough experience with distinguishing between IHC 0 and IHC 1+ or between true IHC 0 and some membrane staining. "Those results really haven't been significant up until now," he said.

He added that reporting any membrane staining as a potential HER2-ultralow case could cause issues in misidentifying the treatment-eligible population for Enhertu. It is rare to see a true IHC 0 slide, he said, because IHC staining can be prone to artifacts, and the tumor tissue can be heterogeneous. "I worry that now we'll say, 'Let's give the patient the benefit of the doubt and report [the tumor] as ultralow' when it really is HER2 IHC 0," Misialek said.

There are a few tools pathologists can use to confirm HER2 expression in tumor samples that appear to be on the threshold of IHC 0 and IHC 1, Misialek noted. CAP guidelines currently recommend a second review by another pathologist in these situations or examining the case under a high-powered microscope. He hopes that CAP will continue working with ASCO to develop more detailed guidelines, including more specific cutoffs for distinguishing between HER2-low and HER2-ultralow tumors, and consider including these borderline IHC 0 and IHC 1 cases in proficiency testing programs.

Misialek also encouraged oncologists to reach out to pathology colleagues in light of these results from DESTINY-Breast06 and discuss how results should be reported. "It further underscores the pathologist's role in the cancer team" and the importance of "having open dialogue with the other members of the team, specifically the oncologists, and having the oncologist know what reporting criteria is [for HER2 expression] at their institution," he said.